You can get it a the shops in Little India. I didn't know that onion seeds were called kalonji(sp) which is how they are labeled in the shop so you are already ahead of the game. Not sure on specific names for stores but there are several on the south side which carry it.

Not sure where in midtown Toronto you are - but this is available in any Indian grocery store. There are a few in the Don Mills/Eglinton/Laird/Overlea area known as Thorncliffe. There are also some on Lawrence east of Vic Park.Not sure if those are too out of the way, but they will have nigella seeds for sure.